Left: H-band image (4.5′ side length) of the environment of XMMU J0338.8+0021 at z = 1.490. The XMM-Newton detected X-ray emission is shown by the log-spaced blue contours, with the four lowest ones corresponding to significance levels of 2, 2.7, 3.6, 4.6σ above the mean background. The associated overdensity of color-selected red galaxies is displayed by the red contours, showing linearly spaced significance levels of 2–20σ. The large, black circles indicate the 0.5′/1′ radii around the X-ray center; the small black ones mark the spectroscopically confirmed cluster members, with the corresponding IDs. The image was smoothed with a 3 pixel (1.3″) Gaussian kernel. Right: z + H-band color composite image of the core region of the cluster. X-ray contours are displayed in cyan, red galaxies with 2.6 ≤ z − H ≤ 3.5 are marked by red dashed circles, spectroscopic members by white circles and interlopers (i.e. spectroscopically confirmed non-members) by green crosses. The 30″ radius from the X-ray center is indicated by the yellow circle. The white ruler represents the beam size (FWHM) of XMM-Newton at the observed off-axis angle of  ~ 5.6′.